How to retrieve iPhone IDFA from API?

First, you have to ask permission from the user to use their IDFA:

#import <AppTrackingTransparency/AppTrackingTransparency.h> 

[ATTrackingManager requestTrackingAuthorizationWithCompletionHandler:^(ATTrackingManagerAuthorizationStatus status) {
 // Tracking authorization completed. Start loading ads here. 
 }];

This permission flow will only run once, the first time it is called, even if you re-start the app and/or call it again. If you want to answer differently, you'll have to delete the app off the device or simulator completely and reinstall it. Note that iOS simulators return blanked IDFAs (all zeroes) no matter what the answer to the permission flow. See https://developer.apple.com/documentation/apptrackingtransparency for details, including how to customize the message shown to users when asking to track them. Note that many advertising SDKs have their own consent flow calls that you can use.

To actually get the IDFA once you have permission:

#import <AdSupport/ASIdentifierManager.h> 

If you would like to get it as an NSString, use:

[[[ASIdentifierManager sharedManager] advertisingIdentifier] UUIDString]

So your code might look like this:

NSString *idfaString = [[[ASIdentifierManager sharedManager] advertisingIdentifier] UUIDString];
NSLog (@"IDFA: %@", idfaString);

You first have to check if user user has decided to opt out from ad tracking. Only if he allowed it you can use the IDFA.

You can check it by calling isAdvertisingTrackingEnabled method of ASIdentifierManager.

isAdvertisingTrackingEnabled

Check the value of this property before performing any advertising tracking. If the value is NO, use the advertising identifier only for the following purposes: frequency capping, conversion events, estimating the number of unique users, security and fraud detection, and debugging.

The following code snippet shows how to obtain a string value of IDFA.

ObjC

@import AdSupport;

- (NSString *)identifierForAdvertising {
    // Check whether advertising tracking is enabled
    if([[ASIdentifierManager sharedManager] isAdvertisingTrackingEnabled]) {
        NSUUID *identifier = [[ASIdentifierManager sharedManager] advertisingIdentifier];
        return [identifier UUIDString];
    }

    // Get and return IDFA
    return nil;
}

Swift

import AdSupport

func identifierForAdvertising() -> String? {
    // Check whether advertising tracking is enabled
    guard ASIdentifierManager.shared().isAdvertisingTrackingEnabled else {
        return nil
    }

    // Get and return IDFA
    return ASIdentifierManager.shared().advertisingIdentifier.uuidString
}

IDFA - Identifier for Advertising

isAdvertisingTrackingEnabled -> trackingAuthorizationStatus

From iOS v14 Apple deprecated isAdvertisingTrackingEnabled and moved the logic into AppTrackingTransparency Framework. Now user has to grand a permission to read idfa(in the same way as Location permission)

User can control it via:

#iOS 13
#AdSupport
#ASIdentifierManager.shared().isAdvertisingTrackingEnabled
Settings -> Privacy -> Advertising -> Limit Ad Tracking

#iOS 14
#AppTrackingTransparency
#ATTrackingManager.trackingAuthorizationStatus

#a global flag
Settings -> Privacy -> Tracking -> `Allow Apps to Request to Track` 
#or select an app from list to control every app separately 

#a local flag
Settings -> <app_name> -> Allow Tracking 

Implementation

import AppTrackingTransparency
import AdSupport

func getIDFA() -> String? {
    // Check whether advertising tracking is enabled
    if #available(iOS 14, *) {
        if ATTrackingManager.trackingAuthorizationStatus != ATTrackingManager.AuthorizationStatus.authorized  {
            return nil
        }
    } else {
        if ASIdentifierManager.shared().isAdvertisingTrackingEnabled == false {
            return nil
        }
    }

    return ASIdentifierManager.shared().advertisingIdentifier.uuidString
}

ASIdentifierManager is the official way to garner the Advertising Identification Number from a device running iOS 6+. You can use -[[ASIdentifierManager sharedManager] advertisingIdentifier]; to get it.